1. Field of the Invention
The present invention relates to porous polyolefin membranes. More specifically, it relates to porous polyolefin membranes useful for applications including separation membranes or separators for galvanic cells.
2. Description of the Related Art
Porous plastic membranes have been used in a number of applications including separation membranes for filtration in medicine and industry, separators in galvanic cells and electrolytic capacitors, linings for paper diapers and other hygiene products, and building materials such as construction film and roofing base materials. In particular, porous polyolefin membranes are useful in applications in which the membranes come into contact with organic solvents or alkali or acidic solutions owing to their resistance to these substances.
Known manufacturing processes for porous polyolefin membranes include the following:    (a) Wherein a film consisting of a polyolefin combined with an inorganic filler, such as silica or talc, or an organic filler, such as nylon or polyethylene terephthalate, which are insoluble in the polyolefin, is stretched at least in one direction to form pores at the interfaces between the matrix polymer and filler (“multicomponent stretching” hereinafter).    (b) Wherein a crystalline polypropylene film manufactured at a high draft ratio, subsequently heat-treated as necessary, is stretched at least in one direction to form fibrils between lamellar crystallites, yielding a porous film (“single-component stretching” hereinafter).    (c) Wherein a film obtained by the formation of a mixture of a polyolefin resin with an organic liquid or an inorganic filler undergoes extraction of said organic liquid or inorganic filler, with stretching before or after said extraction, as necessary (“mixture extraction” hereinafter).
The multi-component stretching process, (a) above, may involve the addition of an inorganic filler or organic filler. A substantial quantity of inorganic filler is required, which may impair the properties, pliability, and acid/alkali resistance of the matrix polyolefin. An organic filler not only impairs the properties and pliability of the polyolefin but also presents difficulties in ensuring a fine dispersion in the matrix polymer, thus making it difficult to obtain films with a fine pore diameter or a high porosity.
The single-component extraction process, (b) above, involves film formation at a high draft ratio, prolonged heat treatment in a separate step, and multistage stretching under specific conditions. The sophistication inherent in the process necessitates a long manufacturing interval and suffers from low productivity. Additional problems include difficulty in obtaining films with a high porosity, which is intrinsic to the technique of fibril formation between lamellar crystallites, and low tear resistance of the membrane obtained as a result of stretching a highly oriented crystalline sheet.
The mixture extraction process, (c) above, represents a comparatively complicated method involving an extraction step wherein said organic liquid in the sheet is extracted with an organic solvent, or said inorganic filler with an alkaline solvent, followed by a washing and drying step. The organic liquid, if used, accounts for 40–60% by weight of the sheet, which retards the film forming process and renders stretching difficult. Moreover, the organic liquid tends to coat the rollers and other machine parts of the manufacturing process leading to reduced productivity.
Meanwhile, a porous membrane involving a simple manufacturing process thereof has been disclosed, wherein said membrane comprises of a component A consisting of an ethylene-propylene block copolymer, a component B consisting of a homopolymer or a random copolymer of propylene, and a component C consisting of a low molecular weight polypropylene, with the addition as necessary of a component D consisting of calcium carbonate or a component E consisting of a nucleation agent for beta spherulites (see e.g. Reference 1). In addition, a porous film has been disclosed consisting of a crystalline polyolefin resin composed of an ethylene-propylene block copolymer solely or, as necessary, in combination with polypropylene or polyethylene, to which a mineral oil or an ester compound with a boiling point up to 100° C. has been incorporated (see e.g. Reference 2).
These methods intend to improve the poor porosity and air permeability of pure ethylene-propylene block copolymer films by using additional components. However, a multicomponent system requires uniform distribution of those components to generate a uniform porous film. Furthermore, the pores thus formed in the membrane tend to consist of large diameters, which lead to difficulties in obtaining thin membranes, or membranes with a high porosity or high air/moisture permeability. Therefore, porous membranes manufactured according to these methods are not suitable for use as battery separators which require fine pores, or precision filters which require high porosity and air permeability.
Reference 1:
U.S. Pat. No. 5,176,953
Reference 2:
JP 8-208862 A (1996)
Problems Addressed by the Invention
The purpose of the present invention is, therefore, to solve the problems mentioned above in relation to conventional porous polyolefin membranes. More specifically, its purpose is to provide a porous polyolefin membrane having a simple resin composition, which facilitates uniform distribution of the components during the manufacturing process, resulting in the yield of fine pores and a high porosity.
Means to Solution of the Problems
The inventors have found that the problems mentioned above can be solved by the manufacture of a porous polyolefin membrane formed by melting and kneading a specific resin (C) which composition containing a polyolefin resin consisting of a crystalline polypropylene (A) and a propylene-α-olefin copolymer (B), dispersed in said crystalline polypropylene (A), to obtain a film-shaped melt, and forming a membrane from said melt in specific conditions, followed by stretching of the said membrane at least in one direction, which contains continuous pores in the region consisting of said copolymer (B). The present invention is based on this finding. “Continuous pores”, as mentioned in this specification, describes pores formed in a continuous manner in the copolymer (B) region, which eventually connects the two surfaces of the porous membrane.